The Emperorshell is an incredible creature living on the 6th layer of the Abyss, presumably first encountered by the White Whistle Delver Lyza, "The Annihilator", and having received its name from her.
It is a lizard that appears to have adapted to the hazardous habitat of the Capital of the Unreturned, growing up in their eggshell and using it as a shield to keep them safe even from the "iron rain" of the 6th layer. They are quite ferocious, but edible, and can be eaten without too much preparation.
According to Lyza, they will continue to grow endlessly for as long as they live and they appear to have a ridiculously long lifespan. The legendary Delver claims to have encountered one with a towering frame of 30 meters. This elder emperorshell was strong enough that even the White Whistle Delver Lyza, cherished for her phenomenal fighting ability, failed to take it down, and she named it "Giant Emperorshell" following her unsuccessful hunting.